As used in this specification, the following terms have the associated meanings:
The term "modification ratio" or "MR" means the ratio of the radius R.sub.2 of the circumscribed circle to the radius R.sub.1 of the inscribed circle as shown in FIG. 3.
The term "arm angle" or "A" is the angle formed by extension of the sides of an arm as shown in FIG. 3.
The term "trilobal" refers to fibers having three lobes and a modification ratio greater than 1. These trilobal fibers have lobes substantially without convex curves where such convex curves are connected cusps along the contour of the lobe.
The term "pointed lobe trilobal" refers to a trilobal fiber cross-section where each lobe is defined with reference to a single arm angle and each arm angle is greater than 56.times.MR.sup.-1.5.
The term "standard trilobal" refers to a trilobal fiber cross-section where each lobe is defined with reference to a single arm angle and each arm angle is less than or equal to 56.times.MR.sup.-1.5.
The term "triangular lobal" refers to a fiber cross-section having lobes and a base portion. The base portion without the lobes is approximately triangular. The lobes are present at the apexes (tips) of the triangle. Exemplary triangular lobal fibers are the smaller fibers shown in FIG. 3. Fibers of this cross-section are sometimes referred to as having a "fox cross-section" because the cross-section resembles a fox's head. The triangular lobal cross-section is characterized by three lobes and two arm angles, A.sub.0 and A.sub.1. A.sub.0 typically may range from 0.degree. to 25.degree. and A.sub.1 typically may range from 60.degree. to 110.degree..
The term "fiber" refers to both filaments (strands of indefinite or continuous length) and staple (strands of short and definite length).
In the production of fiber for use in carpets, it is desirable to produce fiber that will provide carpet having a pleasing appearance with respect to cover, firmness and luster. It is known to increase carpet cover per carpet weight by using trilobal fibers having a high modification ratio. However, increasing the modification ratio usually results in a reduction in luster. The present invention involves mixed filament yarns which provide a surprisingly excellent balance of qualities when used in carpet.
U.S. Pat. No. Re 29,352 to Newton discloses the broad concept of mixing fibers and is primarily directed to an interlacing method for forming heather fabrics.
U.S. Pat. No. 3,220,173 to Pitzl describes trilobal filaments having a modification ratio falling within a range around a mean modification ratio in the range of 1.4 to 2.5. These filaments are not true mixed filaments, but rather a preset range of cross-sections around a mean filament shape.
U.S. Pat. No. 3,994,122 to Shah describes a blend of crimped staple fibers for use in carpet. The blend comprises two components or groups of trilobal fibers of the same denlet, one group having a modification ratio between 1.6 and 1.9, and the other group having a modification ratio between 2.2 and 2.5. While the blend of fibers is described as providing improved appearance when compared to carpets produced from fibers of either component alone, there is still substantial room for further improvements along these lines.
U.S. Pat. No. 4,001,369, also to Shah, describes a process for co-spinning the trilobal filamentary yarn described in U.S. Pat. No. 3,994,122.
U.S. Pat. No. 4,472,481 to Shooks, Jr. et al. describes a trilobal fiber blend having 70% to 90% by weight of a crimped trilobal polyamide fiber with a modification ratio between 3 and 3.4 and a denlet from 16 to 2,4, and 30% to 10% by weight of crimped trilobal polyamide fibers with a modification ratio ranging from 1.7 to 2.4 and a denlet ranging from 6 to 12.
U.S. Pat. Nos. 5,208,106 and 5,108,838, both to Tung, describe lobal fiber cross-sections where the lobes have substantially convex curves connected by cusps along the contour of each lobe.
U.S. Pat. No. 4,770,938 to Peterson describes a trilobal fiber having an axially extending hole in each lobe. The total cross-sectional void area is 5 to 12% and arm angles of about 15.degree. to 45.degree..
U.S. Pat. No. 4,492,731 to Bankar et al. describes trilobal carpet fibers in a specified range of modification ratio and related arm angles.
U.S. Pat. No. 5,208,107 to Yeh et al. describes a trilobal fiber with a single axially extending central void. The cross-sectional void area is 3-10%. The modification ratio is between 3 and 10 and the arm angle is between about 7.degree. and about 35.degree..
While blends of fibers have been shown to provide various results, there is still room for improvement in the quality of appearance and texture of carpet yams.